Sewerage refers to the collection, treatment and disposal of liquid waste. Sewage systems include all the physical structures required for collection, treatment and disposal of the wastes. In other words, discharged waste water's that are collected in large sewerage networks, transporting the waste from the site of production to the site of treatment comprise Sewage treatment networks (Sewerage system).

SEWAGE:

Sewage consists of liquid wastes produced in residences, commercial establishments and institutions; Liquid Wastes discharged from industries; and any subsurface, surface or storm-water which enters the sewer. Hence basically sewage contains three components:

Sanitary or domestic sewage

Industrial wastes

Infiltration, Inflow and storm-water.

SEWER:

A Sewer is a pipe or conduit, normally closed, flowing full or partially full, which carries sewage. Classification based on use of Sewers or Sewage systems can be done as follows:

Sanitary Sewers: These sewers transport domestic sewage, industrial wastes to the treatment plant. Storm-water does not normally enter these sewers except through joints, manhole covers and defaults in the system.

Storm sewers: These carry the surface and storm-water passing through or generated in the area that they usually serve.

Combined Sewers: These sewers carry all types of wastes - domestic sewage, industrial wastes and storm water in the same conduit.

For Sewage Treatment Systems, large civil and infra-structural investments (like electricity supply) are required, generally paid by the authorities from the income tax collected from the citizens. In order to prevent clogging of sewers, a substantial amount of clean, and often potable water is required, which subsequently results in a dilution of the concentrated waste. The treatment systems, which need to be developed to treat the huge flows of diluted waste water, are extremely expensive and consume large amounts of energy. Moreover, the installed sewerage systems often suffer from lack of maintenance leading to huge losses of waste water (particularly in the developing countries).

A sewage network is a transportation system for human excreta and/or industrial wastes to a central discharge point and/or treatment system, with valuable drinking water as the transport medium. The water demand for such sewer system is extremely high and absurd in conditions where safe drinking water is hardly available, like those prevalent in most cities and towns in India. Also from the environmental engineering point of view, the generally applied centralised sanitation concept can be questioned, because concentrated wastes are relatively easy to manage while management of diluted wastes requires large investments because of the increased quantity of wastewater to be treated.

Many cities are served with sewer systems though the coverage and the treatment given to domestic and industrial wastewaters before discharge into surface water bodies is often inadequate.

1.DEGREE OF TREATMENT TO BE GIVEN TO THE WASTEWATER:

The degree of treatment in most cases is decided by the regulatory authorities [which in the case of India is the Central Pollution Control Board (CPCB) along with the various State Pollution Control Boards (SPCB's)] and the application for which the final effluent from the treatment plant is to be utilised/ disposal method used for the final effluent.

2. DESIGN PERIOD:

The sewage treatment system is normally designed to meet the requirements for a period of 30 years or more after the completion of construction activities. The system should not be under-loaded in the initial years nor be over-loaded in the dying stages. A provision has also to be made for installation of civil structures at any later date depending upon the requirement.

3. POPULATION TO BE SERVED:

A thorough estimate of the population to be served by the treatment system throughout its life period is necessary for the estimation of the quantity of sewage treatment that would require treatment.

4. SEWAGE FLOWS:

The quantity and quality (characteristics) of sewage show a marked hourly variation and hence peak, average and minimum flows are important considerations from design point of view. The process loadings in the sewage treatment plant are based on daily average flows and the average characteristics are determined from a 24 hour weighted composite sample.

5. POPULATION EQUIVALENT:

The population equivalent is useful for conversion of the contribution of wastes from industrial establishments as compared to the domestic wastes, for acceptance in the sewage system. The average daily per capita contribution of suspended solids and BOD5 are 90 grams and 45 grams respectively, for estimation of population equivalent in Indian conditions.

Sewage treatment through water hyacinth

Researchers discovered that water hyacinths thrive on sewage by absorbing and digesting nutrients and minerals from wastewater. Thus a means of purifying water at a fraction of the cost of a conventional sewage treatment facility had been found. Some added benefits of the hyacinths are that after routine harvesting, they can be used as fertilizer, high protein animal feed, or as a source of energy. Today, dozens of small southern towns are now using water hyacinths as their primary method for treating wastewater. Researcher built a one million gallon per day plant for service in 1984. It uses water hyacinths in a hybrid aquatic plant/microbial filter. In addition, Disneyworld's EPCOT Center now operates a 50,000 to 350,000 gallon a day water treatment facility to explore advanced applications of hyacinths. The limitations of hyacinths to warm climates challenged the Stennis Space Center team to continue its research with an artificial marsh filtering system that employs a combination of sewage-digesting microbes and pollutant-absorbing plants such as bulrushes, reeds, and canna lilies. Read more....

CPCB to bring about new norms for sewage treatment plants as government takes steps to clean up rivers.

A trickling filter, also called trickling biofilter, biofilter, biological filter and biological trickling filter, is a fixed-bed, biological reactor that operates under (mostly) aerobic conditions. Pre-settled wastewater is continuously ‘trickled’ or sprayed over the filter. As the water migrates through the pores of the filter, organics are aerobically degraded by the biofilm covering the filter material.

Trickling filters are conventional aerobic biological wastewater treatment units, such as active sludge systems or rotating biological contactors. The advantage of all these systems is that they are compact (i.e. applicable in densely populated urban settings) and that they efficiently reduce organic matter (JENSSEN et al. 2004).

However, they are high-tech and generally require skilled staff for construction as well as for operation.

The trickling filter consists of a cylindrical tank and is filled with a high specific surface area material, such as rocks, gravel, shredded PVC bottles, or special pre-formed plastic filter media. A high specific surface provides a large area for biofilm formation. Organisms that grow in the thin biofilm over the surface of the media oxidize the organic load in the wastewater to carbon dioxide and water, while generating new biomass. This happens mainly in the outer part of the slime layer, which is generally of 0.1 to 0.2 mm thickness (U.S.EPA 2000).

The incoming pre-treated wastewater is ‘trickled’ over the filter, e.g., with the use of a rotating sprinkler. In this way, the filter media goes through cycles of being dosed and exposed to air. However, oxygen is depleted within the biomass and the inner layers may be anoxic or anaerobic.

The word filter is somehow misleading, as physical straining of solids is only marginal. The removal of organic substances occurs by use of bacterial action (UNEP & MURDOCH UNIVERSITY 2004). Therefore trickling filters are also called bio- or biological filters to emphasize that the filtration. Fixed film biological treatment are also used in other common treatment processes such rotating biological contactors of fixed film activated sludge systems.

Filter media

The ideal filter material is low-cost and durable, has a high surface to volume ratio, is light, and allows air to circulate. Whenever it is available, crushed rock or gravel is the cheapest option. Specially manufactured plastic media, such as corrugated plastic sheets or hollow plastic cylinders, that optimise surface area for bacteria to attach free movement of air are also available (UNEP & MURDOCH UNIVERSITY 2004). The particles should be uniform and 95% of them should have a diameter between 7 and 10 cm. A material with a specific surface area between 45 and 60 m2/m3 for rocks and 90 to 150 m2/m3 for plastic packing is normally used. Larger pores (as in plastic packing) are less prone to clogging and provide for good air circulation. Primary treatment is also essential to prevent clogging and to ensure efficient treatment.

Treatment Capacity

Trickling filters are designed primarily for BOD removal. Treatment performances depend on wastewater characteristics, hydraulic and organic loading, medium type, maintenance of optimal dissolved oxygen levels, and recirculation rates (UNEP 2004). A BOD reduction of 60 to 85 % can be expected with loading rates of 1 kg BOD/m3/day (SASSE & BORDA 1998; U.S.EPA 2000; UNEP 2004; WSP 2008;). Bacterial reductions have been reported to be 1 to 2 logs of faecal Coliforms (UNEP 2004), respectively 60 to 90 % of total Coliforms (WSP 2008). Physical adsorption of virus on the biofilm or elimination by predation are additional factors in pathogen elimination in trickling filters (STRAUSS n.y.). Total suspended solids (TSS) removal is expected to be very low (due to the down-flow regime) and pre-settling as well as removal of the solids from the effluent is recommended. Because aerobic bacteria convert ammonia to nitrate (WSP 2008), some nitrification can also be achieved, depending on the organic loading rate to the filter, the temperature and the aeration. Total nitrogen removal varies from 0 to 35 % (UNEP 2004; WSP 2008), while phosphorus removal of 10 to 15 % might be expected (UNEP 2004). However, the capacity for nutrient removal of trickling filters depends strongly on the operation conditions, and while some sources indicate a high removal of ammonia (U.S. EPA 2000) other indicate no capacity of trickling filters for nutrients (UNEP et al. 2004).

Health Aspects/Acceptance

Odour and fly problems require that the filter be built away from homes and businesses. Appropriate measures must be taken for pre- and primary treatment (settling), secondary treatment (eventually final clarifier), effluent discharge and solids treatment, all of which can still pose health risks.

Applicability

This technology can only be used following primary clarification since high solids loading will cause the filter to clog. Since trickling filter only receive liquid waste, they are not suitable where water is scarce or unreliable. Moreover, trickling filters require some specific material (i.e. pumps and replacement parts) and skilled design and maintenance (SASSE & BORDA 1998). A low-energy (gravity) trickling system can be designed, but in general, a continuous supply of power and wastewater is required. However, energy requirement for operating a trickling filter is less than for an activated sludge process (UNEP & MURDOCH UNIVERSITY 2004) or aerated lagoons.

Compared to other technologies (e.g., Waste Stabilization Ponds), trickling filters are compact, although they are still best suited for peri-urban or large, rural settlements..

Trickling filters can treat domestic blackwater or brownwater, greywater or any other biodegradable effluent. They are typically applied as post-treatment for upflow anaerobic sludge blanket reactors or for further treatment after activated sludge treatment. In any case, primary sedimentation (see also septic tanks or pre treatment) is compulsory to avoid clogging of the filter bed and a secondary clarification step and post-treatment of excess sludge (e.g. in sedimentation ponds, unplanted drying beds, planted drying beds or anaerobic digesters) is also compulsory (WSP 2008).

Trickling filters can be built in almost all environments, but special adaptations for cold climates are required. Proper insulation, reduced effluent recirculation, and improved distribution techniques can lessen the impact of cold temperatures (UNEP 2004).

Advantages

Can be operated at a range of organic and hydraulic loading rates

Resistant to shock loadings

Efficient nitrification (ammonium oxidation)

High effluent quality in terms of BOD and suspended solids removal; in combination with a primary and tertiary treatment also in terms of pathogens

Small land area required compared to constructed wetlands

Disadvantages

High capital costs

Requires expert design and construction, particularly, the dosing system

Requires operation and maintenance by skilled personnel

Requires a constant source of electricity and constant wastewater flow

Wastewater filtration is often part of the tertiary treatment process that involves the final removal of suspended particles from water that has passed through both the primary and secondary treatment phases and immediately precedes disinfection.

Biological filtration or biofiltration is one water treatment process that can effectively remove organic matter that is not able to be removed from water and biologically treated sewage effluent in conventional sewage treatment (Carlson and Amy,1998). The biological filter mainly relies on the activities of the community of micro-organisms that are attached on to the filler media. The activities of microbes determine the performance of biological filtration. Microbes oxidize organic matters in water to produce energy and therefore available nutrients sources in feed water is essential for their development. In addition, the parameters such as hydraulic loading rate, back washing techniques, temperature and pH etc. can affect the growth of biomass onto washing granular activated carbon (GAC) in the biofilter. Moreover, biological filtration is economical and safe for environment. Biofiltration is more suitable than other treatment methods in terms of removing organic matter.

Any type of filter with an attached biomass on the filter media can be defined as a biofilter. It can be the trickling filter in the wastewater treatment plant, or horizontal rock filter in a polluted stream, or granular activated carbon (GAC) or sand filter in a water treatment plant. Biofilter has been successfully used for air, water, and wastewater treatment. Originally, biofilter was developed using rock or slag as the filter media, however at present, several types and shapes of plastic media are also used. There are a number of small package treatment plants currently available in the market where different shaped plastic materials are packed as filter media and are mainly used for treating a small amount of wastewater (e.g from household or hotel scale). The basic principle in a biofilter is the biodegradations of pollutants by the micro-organisms attached onto the filter media.

In nature, bacteria and other small organisms in water consume organic matter in sewage, turning it into new bacterial cells, carbon dioxide, and other by-products. The bacteria normally present in water must have oxygen to do their part in breaking down the sewage. In the 1920s, scientists observed that these natural processes could be contained and accelerated in systems to remove organic material from wastewater. With the addition of oxygen to wastewater, masses of microorganisms grew and rapidly metabolized organic pollutants. Any excess microbiological growth could be removed from the wastewater by physical processes.

Physical processes were some of the earliest methods to remove solids from wastewater, usually by passing wastewater through screens to remove debris and solids. In addition, solids that are heavier than water will settle out from wastewater by gravity. Particles with entrapped air float to the top of water and can also be removed.

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